The invention generally relates to the upgrading of soft or weak soil areas having low shear or bearing strength, such as alluvial soil or hydraulic fill areas. More particularly, the present invention relates to improvements relating to the treatment of soil masses for building foundations and like structures through the construction of compacted granular or stone columns in situ or in soil masses.
Stone columns, as the name implies, are simply vertical columns of compacted crushed stone, gravel or sand which extend through a deposit of soft material or soil to be strengthened. Normally a number of these densely compacted granular material columns are produced beneath the site for the intended construction project. These columns serve to stabilize the soil, resulting in considerable vertical load capacity and improved shear resistance in the soil mass.
Stone column applications have included soil stabilization to limit settlement under reinforced earth walls, tank farms, dam and highway embankments, bridge abutments, and buildings. Another application is the stabilization and prevention of landslides. Stone columns also function as efficient gravel drains in providing a path for relief of excess pore water pressures, thus preventing linquefaction during an earthquake.
There are a number of well known methods for the formation of stone columns in the ground. One such common method is the use of a special vibrator, sometimes known as a Vibroflot, which expels water from its body as it sinks into the ground, thus forming a hole. The hole, held open by water pressure, is then filled with stone and the stone is compacted into the ground in stages using the vibrator. A stone column is thus formed in the ground which serves to strengthen the soil and also provides a draining path which is beneficial to the rapid consolidation of the ground as structural loads are subsequently applied. An example of this method is described in U.S. Pat. No. 4,397,588 for METHOD OF CONSTRUCTING A COMPACTED GRANULAR OR STONE COLUMN IN SOIL MASSES AND APPARATUS THEREFORE.
Utilization of this method produces very large quantities of silt laden effluent which must be disposed of. Disposal of this effluent is difficult and expensive under the best of conditions, and virtually prohibitive at environmentally sensitive locations. Consequently, most column installation with Vibroflots now makes use of ancillary bottom-feed equipment which provides a feed pipe to the tip of the Vibroflot. Stone with compressed air is fed through this pipe to the tip of the vibrator, thus eliminating the need for water. Although production by this method is much slower, savings in effluent disposal usually more than offset the additional cost.
Other known methods for the formation of stone columns utilize an elongated hollow tube or pipe which is penetrated into the ground, usually with the aid of vibration. Crushed stone or other granular material is then charged into the tube and as this particulate material is fed to the bottom of the tube and discharged the discharged and particulate material is compacted through the application of vertically applied forces either by repeatedly raising and lowering the pipe as it is withdrawn from the ground or by a reciprocating compactor mounted in the tube.
Examples of these methods for producing stone columns are illustrated in the following U.S. Pat. Nos. 3,648,467; 3,720,063; 3,772,892; 3,808,822; 4,126,007; 4,487,524; and 4,730,954.
Most of these prior art methods make use of an equipment withdrawal and repenetration sequence. That is, particulate material is deposited in the bore as the probe is withdrawn. This freshly placed material is then compacted and forced outward into the native soil by repenetration of the probe. A disadvantage of this sequence is that large amounts of soft native soil are dragged down with the probe into the column, resulting in considerable contamination and mixing of the native soil into the column. Such contamination and mixing tends to weaken the column as well as to lower its permeability.
Another problem with all of the above-identified prior art methods is that it is difficult to provide adequate quality control techniques in stone column construction.
A good quality stone column is one which performs efficiently at a given replacement ratio and it is generally agreed that such a column must be constructed of material which has a large angle of internal friction. This material should be tightly compressed into, and thus supported by, the in situ soil. Present practice is to assume that motor power consumption achieved during column repenetration provides a measure of this confinement. However, earth reaction forces significantly affect the behavior of the equipment which is utilized to install the stone column and thus motor power consumption cannot completely specify conditions. This is true not only with regard to the first above-mentioned technique utilizing a laterally vibrating probe, but this is also true with the use of the techniques which utilize an elongated vertical tube, as the forces utilized to downwardly expel the granular material from the bottom of the hollow tube does not provide a measurement of the degree of lateral compression of the granular material within the column being constructed.
This is so because the techniques employed to expel the granular material from the bottom end of the hollow tubular structures function to force the granular material downwardly out of the bottom end of the tube and therefore to date no adequate method has been provided for adequately forcing the granular material outward in a radial direction away from the bottom of the tube and in addition to provide a means for adequately measuring the applied forces required to accomplish this radial compaction of the granular material.
In actuality, there is no relationship between the computed centrifugal force that the Vibroflot provides, or the outward radial forces that the computed downward expulsion force of particulate material that the elongated tubular member provides during repenetration, and the amount of force with which the stones or granular material is forced or propelled into the in situ soil during stone column installation. In reality, these forces for creating the particulate or stone columns of the prior art methods and structures bear very little relationship to the actual force which exists between the apparatus and the soil within which the stone column is being constructed.
Accordingly, a measurement of motor power consumption which energizes the apparatus of the prior art for applying these compaction forces does not provide any adequate measure of the applied forces radially imposed on the in situ soil and the particulate material utilized to construct the particulate or stone column. In addition, because prior art devices apply outward forces due only to internal shear occurring in the column when driving the probe into the stone during repenetration, sufficient radial compaction forces cannot be provided and adequately controlled for different given in situ soil conditions in order to provide predetermined radial displacement of the column.
In addition, because of the way compaction forces are applied, all of the above-identified methods for installing granular or stone columns have a relatively slow production rate. For example, with the best of the above-identified bottom feed methods, one can normally install 300 to 350 feet of stone column per day for a single rig adapted for installing the columns in very soft soil. Perhaps even at ideal rates and conditions, a rate of 400 feet per day might be obtainable. However, it is a principal object of the present invention to provide an apparatus and method that will at least double this production rate for the same column construction such that possibly 1,000 feet of column may be produced by one rig per day in the same given period of production time.
It is also a principal object of the present invention to eliminate the disadvantages of the above-mentioned prior art apparatus and methods for constructing stone columns in situ and to produce such columns at a reduced cost with improved effectiveness and with improved quality control over construction. It is a further object and advantage of the present invention to provide an apparatus and method for constructing such granular or stone columns in situ under such conditions which eliminate intermixing and contamination of the column with native soil, and which can force the sand or stone radially outward in a precisely controlled and regulated manner such that the measurement of this force is in direct relationship to the actual force which exists between the equipment installing the stone or granular column and the soil. Such conditions are ideal to produce soil fracturing or vertical cracks which provide drainage channels to reduce time for reconsolidation with additional soil improvement during and after installation (K. R. Massarch, "New Aspects of Soil Fracturing in Clay," Jour. of the Geot. Engr. Div., ASCE, Vol. 104, No. GT8, August, 1978).